Storage layer including arsenic, selenium and sulphur



May 3, 1966 v. J. SANTILLI ETAL STORAGE LAYER INCLUDING ARSENIC, SELENIUM AND SULPHUR Filed Feb. 1, 1965 2 Sheets-Sheet l Fig.l.

0 -w O I 5 9 T F s UE 0 T 8 M R C Q E m T P N F E M D m O R E E T W 6 U TT T SLN A W U 0 O W JS A D E M w P 3 AR M E A n. M T x 9 U E W S H m L E l- O O O O O o o o O 4 3 2 l I NVENTORS Vincent J. Sonfl lli Hurry Shubonowi'r and Albert J. Cornish BY K AT TORNEQ iii/ 2M V y 1966 v. J. SANTILLI ETAL 3,249,783

, SELENIUM AND SULPHUR STORAGE LAYER INCLUDING ARSENIC Filed Feb.

2 Sheets-Sheet 2 Fig.3.

INTEGRATION TIME FOR INITIAL PATTERN 45 SEC.

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zOEAjOmmm STORAGE TIME MINUTES RESOLUTION VS STORAGE TIME Fig.4.

Eg =34V a -23v Eg BEST FOCUS O O 2 I TIME MINUTES RESOLUTION VS INTEGRATION TIME United States Patent 3,249,733 STURAGE LAYER INCLUDING ARSENIC,

SELENEUM AND SULPHUR Vincent J. Santilli, Coming, and Harry Shabanowitz, Elmira, N.Y., and Albert J. Cornish, Monroeville, Pa., assignors to Westinghouse Electric (Iorporation, Pittsburgh, Pa, a corporation of Pennsylvania Filed Feb. 1, 1963, Ser; No. 255,607

' 6 (Ilaims. (Cl. 313-65) This invention relates to a storage device and more particularly to a target member for'said storage device and a method of manufacturing said target member.

A storage device is one in which there is provided a storage electrode for storing signals of variable intensity and then providing for read out of the stored information at a later time. The device normally consists of the -dielectric storage target member in an evacuated envelope upon which is established a pattern of charge representative of the stored pattern or image. The charge pattern is normally established by directing an electron beam or electromagnetic radiation, such as visible light, onto the storage surface.

In a storage tube of the type described above, it is desirable in many applications to perform the operation of read out of the charge image or pattern without the charge image being destructively read out and thus permit read out of the information for as many times as desired. This is referred to as a multicopy type of storage tube. It is also desirable in this type of tube that the information also include half tones, that is signals between a maximum and a minimum value, in all copies.

At the present time there are at least three types of storage devices that provide multicopy read out with half tones. These three devices are the metrechom, the transmission type storage display tube and a storage device as described in US. Patent 3,046,431, issued July 24, 1962, entitled, Storage System, by J. F. Nicholson and assigned to the same assignee as this invention. This invention is particularly directed to an improved target structure which exhibits these effects and may be incorporated into a storage system such as described in the above-mentioned patent.

In the storage device described in the abovementioned patent, a photoconductive pickup storage tube is described in one embodiment. The device consists essentially of a target formed with a light transmissive, electrically conductive coating supported on the face plate portion of the envelope and a layer of arsenic and selenium deposited on the conductive coating with a layer of antimony trisulfide deposited on the arsenic and selenium layer. This target structure is mounted within an evacuated envelope and the exposed surface of the antimony trisulfide layer faces an electron gun assembly. The electron gun assembly generates a low velocity type beam and provides a pencil-like reading electron beam substantially normal to the surface of the antimony trisulfide. The electrons within the reading beam approach the target with very low energies, normally below the first crossover potential of the target surface. These electrons are deposited on the photoconductive surface and drive the surface to substantially the cathode potential of the reading electron beam gun. When this potential, normally referred to as an equilibrium potential, is reached, the remaining electrons in the electron beam are reflected back toward the electron gun assembly.

The electrically conductive layer or signal back plate of the target is held at a potential of about 10 volts positive with respect to the equilibrium potential on the exposed surface of the antimony trisulfide layer. In this manner, there is established a potential gradient across the two layers of material. Due to the photoconductive properties of the target material used, light directed onto the photoconductive materials will cause a charge to be estabished on the scan surface of the target. This change in charge or potential will be toward the potential of the electrically conductive back plate. The electron beam scanning or reading the target areas illuminated by the light will derive a signal from an output circuit connected through the signal plate but will not destructively read out or restore the surface to the equilibrium potential. In fact, it is found that by keeping the scan beam turned, the stored information on the target member will be retained and a plurality of copies may be obtained. The exposure of the pickup tube to the scene can be cut off after a short time or the exposure to the scene can be continued. By continued exposure, it is found that the signal on the target will integrate and build up to a larger signal. If desired, the pattern written on the target can be read out for a considerable length of time after exposure such as 5 seconds to one hour.

In addition to light stimulation or writing as indicated above, information may be written onto the target structure by an electron beam scanning the exposed surface of the photoconductive layer. The writing electron beam source may also be positioned on the opposite side of the target with respect to the reading beam and by the use of electrons of adequate energy may penetrate the electrically conductive layer, induce conductivity Within the arsenic and selenium layer and the antimony trisulfide layer by electron -bombardment induced conductivity. An electron bombardment induced conductivity type of device is described in US. Patent 2,900,555 and assigned to the same assignee as this invention. In addition, US. application Serial No. 855,889, filed November 27, 1959, now Patent No. 3,148,297, entitled, Storage System, by J. Nicholson et a1. and assigned to the present assignee describes this type of assembly.

The length of time or multicopy read out may be rnodified by modifying the amount of potential across the.

target such as varying the signal back plate potential. In addition, the stored image on the target may be removed by exposing the target to a uniform high level light illumination or by turning the scan read beam off for a predetermined period of time of about one minute.

Experience with the device described in US. Patent 3,046,431, has indicated the desirability of providing a target of increased sensitivity, simplified structure and increased temperature operating range.

It is, therefore, an object of this invention to provide an improved radiation sensitive storage electrode.

It is another object to provide an improved storage electrode.

It is another object to provide an improved target for use in electron bombardment induced conductivity type of tube.

It is another object to provide an improved radiation sensitive target capable of withstanding relatively high temperatures.

It is another object to provide a storage device having along period of continuous read out.

In accordance with our invention, we provide a storage target electrode by evaporatlng an admlxture of arsenic and selenium with the addition of sulfur in predetermined proportions to provide a single layer radiation sensitive storage electrode.

Further objects and advantages of the invention will become apparent as the following description proceeds. The features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of the description.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIGURE 1 is a view in section of a pickup tube embodying the teachings of this invention;

FIG. 2 is an enlarged sectional view of the target schematically shown in FIG. 1;

FIG. '3 is a graphical representation indicating the resolution in television lines with regard to storage time in minutes;

FIG. 4 is a graphical representation illustrating the resolution in television lines plotted against the time in minutes with regard to integration time; and

FIG. 5 is a graphical representation of the resolution in television lines plotted with respect to temperature in degrees centigrade.

Referring now to FIG. 1, a pickup storage tube is illustrated. The envelope, electron gun, scanning and deflection system are similar to those utilized in the conventional type of pickup-tube known as the vidicon. The vidicon tube is a well known television pickup tube for producing a video signal from a scene for transmission to another location.

The tube shown in FIG. 1 comprises an envelope of a suitable material such as glass. One end of the envelope 10 is closed by an end wall or face plate portion 12 through which the electromagnetic waves, such as visible light from the scene viewed, enter and energize atarget member 14. The face plate 12 is also of a suitable material transmissive to a radiation from the scene. Glass is a suitable material for viewing scenes of visible light. The interior surface of the face plate 12 is provided with an electrically conductive coating 16 which is also transmissive to radiations from the scene. A suitable material for the coating 16 is stannic-oxide. The input screen or storage member 14 consists of a layer of an evaporated mixture of arsenic, selenium and sulfur deposited on the electrically conductive coating 16. The electrically conductive coating 16 is connected to the exterior of the envelope 10 by a lead-in 60. The lead-in 60 is connected to one terminal of a resistor 62 which is in turn connected to a voltage source 64. The voltage source 64 supplies a variable potential, in the range of about 2 to 50 volts to the coating 16. v

The other end of the tube envelope 10 provides the base of the tube through which various leads (not shown) enter into the tube for applying suit-able potentials to the other electrodes therein. The tube is provided with the necessary well known components needed to produce and control a pencil-like beam of electrons to scan the target 14 in a point-by-point manner. An electron gun 17 is provided'within the tube adjacent the base to generate and form the electron beam. The electron gun 17 may be of any suitable design and includes at least a cathode 20, a control grid 28, a screen grid 30 and an accelerating electrode 32 The cathode 20 is comprised of a tubular sleeve 22 closed at one end facing the screen or target 14. The closed end of the sleeve 22 is provided with a suitable thermionic emission coating (not shown) to provide the source of electrons. A heater 26 is provided within the sleeve 2-2 to heat the emissive coating. The electrons emitted from the cathode 20 are formed into an electron beam by the control grid 28 and the screen grid 30. The cathode 20 is connected to a suitable voltage source 34 which may be at ground potential. signals may be connected to the cathode 20 by switch 38 A source 36 of video ates at a similar potential.

.in some applications to permit the image to be written onto the target member by an electron beam rather than light directed onto the target 14. The control grid 28 is supplied with a suitable negative potential of about 45 to volts from a source 40. It may be desirable in some applications to modulate the control grid 28 by connecting the video source 36 to thecontrol grid rather than the cathode 20 in a well known technique. The screen or accelerating grid 30 is supplied with a positive potential of about 300 volts from a source 42. The beam focus electrode 32 is supplied with a positive potential of about 250 volts from a source 44.

A fine mesh 56 is mounted adjacent to the target 14 and is electrically connected to the electrode 32 and oper- The electrons generated by the electron gun 17 are magnetically focussed to a small spot at the target 14 by means of a magnetic field provided by a focus coil 50. An alignment coil 52 may also be provided to correct for misalignment of the electron gun 17. A deflection yoke 54 is provided for scanning the electron beam over the surface of the target 14 in response to suitable voltages applied to the yoke 54 by well known techniques.

In the conventional vidicon, the electron beam may scan the surface of the storage screen either at a high or low velocity. In the more conventional low velocity type of operation, the electrons approach at a velocity below the first crossover potential of the material on the target member. Electrons are deposited on the target and would tend to charge the surface in a negative direction toward the equilibrium potential which approaches the cathode potential of the gun in the specific embodiment shown herein. If operated in the high velocity mode, the electrons strike the surface of the target between the first and second crossover potential and the target surface would tend to charge in a positive direction by the action of the scanning beam. In a case of the low velocity mode operation as described herein, the conductive back plate is connected through a resistance to establish a potential of about 10 volts positive on the signal plate with respect to the equilibrium potential.

Referring now to FIG. 2 for a more detailed description of the target 14, the target 14 is supported on the light transmissive face plate 12. The target 14 consists of an electrically conductive coating 16 of a thickness of about 500 Angstroms and transmissive to the input radiation. The resistance of the material in the layer 16 should be less than 200 ohms per square. The electrically conductive coating 16 may be of a suitable material such as tinoxide and is formed by spraying a solution of tin over the heated support plate 12. It is also possible to evaporate an electrically conductive coating such as gold according to well known and established techniques onto the face plate 12 to provide the conductive coating. After the electrically conductive coating has been provided on the face plate, the structure is placed in a vacuum of about 10- to 10* millimeters of mercury. A boat of a suitable material such as tantalum is charged with about milligrams of As Se S and positioned at a distance of about six inches from the target. The material As Se S may be prepared by adding 1.4982 grams of arsenic, 1.5792 grams of selenium and 0.32066 gram of sulfur. This mixture is placed in a small quartz bulb and sealed off under a vacuum of 10- millimeters of mercury or better and then placed in a furnace. The bulb is heated to a temperature of about 650 C. for a period of about one-half hour and the contents are mixed during this heating operation by shaking, for example. The bulb is then removed from the furnace and cooled rapidly to room temperature and a suitable composition of As Se S is prepared in this manner. The boat containing the As Se S is heated to a temperature of about 500 C. and the material evaporated onto the conductive coating 16. The heating may be accomplished by passing electrical current through the boat. The thickness of the layer evaporated onto the conductive coating is approximately five microns. The tube is then processed in a conventional manner to completion.

FIGS. 3, 4 and 5 illustrate the operation and performance of a tube incorporating an As Se S target. In FIG. 3, the resolution of thedevice versus storage time for a target voltage of 14 volts and an integration time of 45 seconds is shown. Two curves are shown, one with-an illumination of 16 x 10 foot candles and the other with an illumination of 8 x 10- foot candles.

In FIG. 4, several curves are shown with different illumination illustrating the resolution versus integration time. In this specific test the target voltage was held at 7.8 volts.

Another important consideration in this device is the provision of a suitable photoconductor for operating at substantially high tempertaures. The temperature properties of the target are illustrated in FIG. 5 wherein the resolution with a given light input is plotted with respect to temperature.

In the operation of the device, the voltages previously mentioned may be applied to the electrodes and a light image is directed onto the target by suitable focussing means. If the face plate is capped after a given integration time to prevent any further illumination on the target, it is found that the light image provides a charge pattern on the target 14 which does not extinguish as the scan read beam moves over the target 14. The image may be retained on the target and scanned for a period up to two hours with conventional television scanning techniques wherein 30 frames per second are scanned. It is found that the low velocity reading beam does not erase the charge image as in a conventional vidicon, but instead has the property of retaining the charge image on the target member. The material described herein appears to have a band gap which lies between 1.7 and 2.5 electron volts. The dark conductivity would also fall between the measured values of about 10- to 10 ohms/cm.

Although the device described above refers to a particular material composition of As Se S, other similar materials, both stoichiometric and non-stoichiometric, have been tried and operate satisfactory. These materials may range from As Se S to As Se S Specific materials found suitable were As Se S, As Se S As Se S and As Se S The stoichiometric compositions may vary off stoichiometric composition by plus or minus 20 atomic percent. It is found that sulfur may be substituted for selenium in the composition As Se up to 90 atomic percent of selenium and provide a suitable material.

The operation of the device may be varied for certain applications. For example, with a fixed light input and for a given integration time one can vary the length of storage by varying the target voltage. It is also possible to fix the target voltage and integration period and vary the level of the light illumination and thereby alter the storage time of the device. Another possible method is to fix the target voltage and light input and vary the in tegration time.

To erase the stored signal on the target 14, one may 'bias the reading beam to cutoff or reduce the target voltage to prevent the beam landing on the target. This erase operation requires about one minute. If it is desired that the information on the target be erased almost instantaneously, then a uniform high level source of illumination may be directed onto the target 14. For exampic, 20 foot candles for one second will erase the stored signal. Although-we have described this invention in only one specific embodiment in which a light sensitive target has been described, the target member has a wider variety of tube applications as indicated previously. It may be used as an electron bombardment induced conductivity type of target wherein the writing information is directed on the target by means of a scanning electron beam positioned on one side of the target and the information read out from the other side of the target in the conventional manner described above. In this particular application, the persistence may be controlled as indicated above with regard to a light input and, in addition, the persistence may be varied by varying the velocity of the writing electron beam.

It is also possible to utilize an X-ray sensitive phosphor on the input screen. This would permit conversion of an X-ray image into a light image which would be impressed on the storage layer. 'Another important application of this type of storage material is that as described in the copending application ofG. L. Cox, Serial No. 56,572, filed September 16, 1960, now Patent No. 3,124,715, and assigned to the same assignee as this invention.

While there have been shown and described what are presently considered to be the preferred embodiments of the invention, modifications thereof will readily occur to those skilled in the art. It is not desired, therefore, that the invention be limited to the specific arrangements shown and described, and is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of this invention.

We claim as our invention:

1. A storage tube comprising a storage electrode, said storage electrode including a layer derived from an evaporated mixture of arsenic, selenium and sulfur, said layer exhibiting the property of storing an information pattern, including halftones in response to excitation and discharge of said information pattern in a predetermined time in the absence of electron bombardment, said layer also exhibiting the property of retaining said stored pattern for a greater length of time than said predetermined time in response to a constant current of electrons, means for directing excitation onto said storage electron to establish a storage conductivity pattern in said storage electrode and means for directing an electron beam onto said target to retain said stored pattern for a greater length of time than said predetermined time.

2. A storage tube comprising a storage electrode, said storage electrode including a layer derived from an evaporated mixture comprised of As Se in which sulfur is substituted for selenium up to atomic percent, said layer exhibiting the property of storing an information pattern, including halftones in response to excitation and discharge of said information pattern in a predetermined time in the absence of electron bombardment, said layer also exhibiting the property of retaining said stored pattern for a greater length of time than said predetermined time in response to a constant current of electrons, means for directing excitation onto said storage electron to establish a storage conductivity pattern in said storage electrode and means for directing an electron beam onto said target to retain said stored pattern for a greater length of time than said predetermined time.

3. A storage tube comprising a storage electrode, said storage electrode including a layer derived from evaporating a mixture of AS25628, said layer exhibiting the property of storing an information pattern, including halftones in response to excitation and discharge of said information pattern in a predetermined time in the absence of electron bombardment, said layer also exhibiting the property of retaining said stored pattern for a greater length of time than said predetermined time in response to a constant current of electrons, means for directing excitation onto said storage electron to establish a storage conductivity pattern in said storage electrode and means for directing an electron beam onto said target to retain said stored. pattern for a greater length of time than said predetermined time.

4. The storage electrode in claim 3 wherein there is provided up to a 20 atomic percent variation of any element from the stoichiometric composition.

5. A storage tube comprising a storage electrode, said storage electrode including a layer derived from evaporating a mixture within the range from As Se S to As Se S said layer exhibiting the property of storing an information pattern, including halftones in response to excitation and discharge of said information pattern in a predetermined time in the absence of electron bombardment, said layer also exhibiting the property of retaining said stored pattern for a greater length of time than said predetermined time in response to a constant current of electrons, means for directing excitation onto said storage electron to establish a storage conductivity pattern in said storage electrode and means for directing an electron References Cited by the Examiner UNITED STATES PATENTS 7/1962 Nicholson 313-65 9/1964 Schneeberger et al. 313-65 beam onto said target to retain said stored pattern for a 10 GEORGE WESTBY, Primary Examine!- greater length of time than said predetermined time.

R. SEGAL, Assistant Examiner. 

1. A STORAGE TUBE COMPRISING A STORAGE ELECTRODE, SAID STORAGE ELECTRODE INCLUDING A LAYER DERIVED FROM AN EVAPORATED MIXTURE OF ARSENIC, SELENIUM AND SULFUR, SAID LAYER EXHIBITING THE PROPERTY OF STORING AN INFORMATION PATTERN, INCLUDING HALFTONES IN RESPONSE TO EXCITATION AND DISCHARGE OF SAID INFORMATION PATTERN IN A PREDETERMINED TIME IN THE ABSENCE OF ELECTRON BOMBARDMENT, SAID LAYER ALSO EXHIBITING THE PROPERTY OF RETAINING SAID STORED PATTERN FOR A GREATER LENGTH OF TIME THAN SAID PREDETERMINED TIME IN RESPONSE TO A CONSTANT CURRENT OF ELECTRONS, MEANS FOR DIRECTING EXCITATION ONTO SAID STORAGE ELECTRON TO ESTABLISH A STORAGE CONDUCTIVITY PATTERN IN SAID STORAGE ELECTRODE AND MEANS FOR DIRECTING AN ELECTRON BEAM ONTO SAID TARGET TO RETAIN SAID STORED PATTERN FOR A GREATER LENGTH OF TIME THAN SAID PREDETERMINED TIME. 